Electrically-based temperature sensors, such as thermocouples and resistive temperature devices (RTD), are commonly used to measure temperature. There are numerous applications in which the response time, accuracy or immunity from electromagnetic interference (EMI) of these electrically-based temperature sensors is inadequate. Fiber optic temperature sensors have been developed with improved response time and accuracy, and the optical fiber is inherently immune to EMI.
It is known to measure temperature with optical means sensitive to thermally-generated optical radiation. The spectral radiance, L, of a thermal radiation source is expressed in optical power (watts) per unit area (square meters), per unit wavelength (meter) and per unit solid angle (steradian). The relationship between spectral radiance, L, and temperature, T, (on the Kelvin scale) of the emitting source is given by the Planck equation: ##EQU1## .epsilon.=emissivity .lambda.=wavelength in units of meters
C.sub.1 =3.741.times.10.sup.-16 watt meter.sup.2 PA1 C.sub.2 =0.01439 meter Kelvin
The emissivity, .epsilon., is the ratio of the emission efficiency of a source to that of a perfectly-emitting source of the same shape. Emissivity may vary with wavelength. To determine the optical power, P, (in units of watts) collected by an optical fiber, it is necessary to integrate the spectral radiance over the appropriate wavelength interval, .DELTA..lambda.; the effective area, A; and acceptance angle, .omega., of the optical fiber: ##EQU2## where .theta. is the angle between the normals of the emitting and receiving surfaces.
For ease of installation, it is known to use a light guide containing one or more optical fibers to transmit the thermal radiation from the sensed region to the optical detector, as shown, for example, in U.S. Pat. No. 1,894,109. In such an arrangement, it is necessary to know the emissivity of the radiation source in order to determine the temperature. For most materials, the emissivity varies with wavelength and also in time due to such effects as oxidation, contamination and mechanical wear. Undetected changes in emissivity introduce errors in optical temperature measurement techniques.
A way to minimize the effects of emissivity changes is to locate an optical cavity in the region for which the temperature is to be sensed. By suitably choosing the shape of the cavity, a change in the emissivity of the interior surface of the cavity will result in a proportionately-smaller change in the effective emissivity of the cavity aperture. When the emissivity of the cavity approaches unity, it is called a blackbody cavity.
The use of blackbody cavities for optical temperature measurement is described in U.S. Pat. Nos. 3,626,758 and 4,576,486. In the latter patent, the blackbody cavity is formed by coating the end of single-crystal optical fiber for a length 20 times the diameter of the fiber. The variation is the effective emissivity of this cavity was less than 1% over a range of test conditions. Accurate temperature measurements can be made as long as the coating is not perturbed. In practice, the coating is eroded in adverse environments. The coating thickness can be increased or additional layers added for protection, but only with a corresponding increase in the response time of the probe. A further limitation of the coating technique is that it can be applied only to the core of the optical fiber, whereas many optical fibers have an annular cladding surrounding the core as an integral part of the fiber. A further limitation of U.S. Pat. No. 4,576,486 is that the temperature range does not extend below 500.degree. C.
Another example of an optical temperature sensor is disclosed in U.S. Pat. No. 4,362,057. The thermal radiation from an optical fiber itself is employed to make a temperature sensor. Even transparent materials such as glass have a finite emissivity in the near-infrared, producing thermal radiation above 100.degree. C. This self-generated radiation can be detected at the remote end of the fiber and suitable electronics used to convert an optical signal level to a temperature. This approach is applicable to the measurement of temperature over a region having dimensions of inches or feet, but is not suitable for smaller regions.
Improvements to the ruggedness, response time, usable type of optical fibers, temperature range, and spatial resolution of optical temperature sensors, achieved by the present invention, renders them capable of exploitation for temperature sensing and measurement in such industrial applications as combustion systems, petroleum and chemical processing, and locations with high electromagnetic fields.